Since the color characteristics of image input/output devices are varied depending on the device, when certain color image data is shared between different devices, there is a problem that the color combination reproduced by each device may be unmatched even when the color image data is the same. To solve this problem, a color management system (hereinafter referred to as a “CMS”) is utilized to unify the color appearance of color image between different devices.
In the CMS, to excellently reproduce the same color image on plural image input/output devices (e.g., color copy, color monitor, digital camera, color printer and so on), a color signal of each device is converted into a color signal in a color space for color matching, in which the color signal of an input system is converted into the color signal of an output system so that they may be matched in this color space, if possible. Herein, the input system designates a device aimed at the color matching. For example, when the color of a color printer B is matched with the color of a color printer A, the color printer A is the input system, and the color printer B is the output system. Also, the color space for color matching is generally a calorimetric color space, such as CIE/XYZ, CIE/LAB and CIE/LCh (e.g., refer to Japanese Patent Laid-Open No. 7-200814 (patent document 1)).
However, especially in a low-saturation area, an excellent image may not be reproduced by simply matching the colorimetric values. For example, if the ink or toner is employed as a white color signal of the color printer, it is perceived as “color cast”, which makes a bad visual impression, even if the colorimetric values are matched. Therefore, for the white color typically reproduced on the color printer, the color of an image recording medium (printing paper) itself is employed, without depending on the colorimetric value of the input system.
Also, the black color is often reproduced in the maximal density color intrinsic to the device to utilize a dynamic range of the device to the maximum, and the gray of intermediate lightness is typically a value dependent on the device, because the chromaticity is continuously changed from white to black. That is, it is required in some cases that the image is not reproduced to match the colorimetric value, but reproduced in the color intrinsic to the device, depending on the kind of color. And in such cases, in the color space for color matching, for example, it is desired that the color signal corresponding to gray of the input system and the color signal corresponding to gray of the output system, which are different calorimetrically, are matched.
The color space for color matching with the corrected gray line to meet the above requirement (hereinafter referred to as a “color space S”) is employed in which the equal lightness plane is translated so that the color signal of gray containing white and black may become the color signal having the same lightness and representing the achromatic color (saturation=0) in the colorimetric color space based on three attributes of color perception of lightness, saturation and hue.
FIG. 7 is a diagram showing the relationship between such a colorimetric color space and color space S using the equal lightness plane in the CIE/LAB space. In FIG. 7, the abscissa is a*, the ordinate is b*, point P is a point indicating the gray, point P′ is a point indicating calorimetrically achromatic color, 71 is a gamut of the device in the calorimetric color space, and 72 is a gamut of the device in the color space S. As shown in FIG. 7, the color signal in the color space S is translated from the color signal in the calorimetric color space along a vector PP′.
By this translation (parallel displacement), the gray of each device is represented by a point on the same straight line (axis of lightness), and treated as the color of the same chromaticity point. Also, in the color space S, a line segment indicated by the gray of the input system and a line segment indicated by the gray of the output system can be easily matched by compression/extension on the same straight line, and due to this compression/extension, the color signals indicating the while color and the black color of the input system can be matched respectively with the color signals indicating the white color and the black color of the output system. A technique for preventing color cast by moving the gray line was described in Japanese Patent Laid-Open No. 2001-326826 (patent document 2).
However, in the color space S, the color conversion for converting the achromatic color signal has influence not only on the achromatic color but also on chromatic colors, resulting in a new problem, that color signals indicating the same calorimetric values for the chromatic color may be unmatched. For example, in the color space S, the flesh color (skin tones) of each device is reproduced by a different color signal, whereby the skin tones of the input system cannot be suitably reproduced by the output system. FIG. 8 is a diagram showing the equal lightness plane in the CIE/LAB space to explain the conventional color conversion method. In FIG. 8, the abscissa is a*, the ordinate is b*, point Pi is a point indicating the calorimetric value of color on the gray line in the input system, point Po is a point indicating the calorimetric value of color on the gray line in the output system, point P′ is a point indicating calorimetrically achromatic color, point C is a point indicating the colorimetric value of a skin tone color, point Ci′ is a point indicating the color signal in the color space S for the input system corresponding to the color at point C, and point Co′ is a point indicating the color signal in the color space S for the output system corresponding to the color at point C indicating the colorimetric value of flesh color.
With the conventional method as shown in FIG. 8, the color signal in the color space S corresponding to the color on the gray line of each device can be represented by point P′ without regard to the device, but the color signal in the color space S corresponding to the flesh color of each device is different depending on the device, and indicated at point Ci′ in the input system and point Co′ in the output system. Accordingly, even if the color signal in the input system is converted into the color signal in the output system so that they may be matched in the color space S, the flesh color of the input system and the flesh color of the output system may be different, resulting in a less excellent reproduced image.